A modern home potentially has a great number of sophisticated electrical systems, including security systems, audio/video systems, telephone systems, intercom systems, etc. All of these systems require interconnecting wiring. A security system for example, requires wiring between sensors, controllers, and alarm devices. Audio/video systems require a maze of wiring between different active components, as well as wiring to as many as six speakers in a single room. Telephone and intercom systems similarly require wires between stations.
When systems such as these are installed during construction of a new is home, wiring can be installed with little trouble. When adding systems to an existing house, however, installation of required wiring often requires significant effort.
Because of the difficulty of installing interconnecting wiring in an existing home, there are many available products that utilize existing AC power distribution wires or lines in a house for communications of various types. Products such as these work by modulating a signal on the power lines at a frequency that is well above the conventional 60 Hz frequency of electrical power carried by the distribution lines.
The so-called "X10" protocol is popular for providing simple communications between common electrical components such as security components, switchable power receptacles, dimmers, and other power control modules. The X10 system provides basic functionality between command modules and receivers of various types. In general, however, this system is limited to on/off and dimming capabilities.
A variety of other products are also available. Some home intercoms, for example, modulate an analog audio signal on the power lines to provide audio communications between two different rooms in a house, without requiring dedicated wiring. Extension telephones are available that utilize existing power lines rather than requiring the installation of telephone cable. Adapters are also available for transmitting video and stereo audio over existing power lines in a house.
There are a number of different protocols used for communications over existing building wiring. The relatively simple X10 communications protocol is one example. An X10 signal is composed of a series of 5 volt, 121 kHz pulses having a duration of 1 millisecond, positioned at zero crossings of the 60 Hz AC power signal. Each pulse corresponds to a binary 1, and the absence of a pulse corresponds to a binary 0. A single X10 command consists of a 22 bit word obtained from eleven complete cycles of the AC power signal.
All X10 receivers plugged into the household power lines will see all transmitted signals. However, each command carries the address of its transmitter. A receiver responds to only those commands that have the address of the receiver. Thus, control modules such as switch modules can be paired with receiver modules by manually setting both addresses to the same value. Up to 256 addresses are available. Computer interfaces are available for allowing a computer to issue commands to different X10 receivers over home power lines.
More sophisticated protocols have also been used to communicate using existing power lines. Electrical protocols in most such systems use a modulation carrier that is significantly higher in frequency than 60 Hz. Data formatting in the more sophisticated systems is similar or identical to networking protocols, in which discrete packets of digital information are sent from an originating device to a destination device using a common carrier channel or frequency. To send analog information, an analog signal is digitized and embedded in the packets.
Simple control information such as used in X10 systems requires only a relatively low data bandwidth. Transmitters and receivers capable of such a low bandwidth are fairly inexpensive. Increasingly, however, there are other applications where higher bandwidth is necessary. For example, transmitting many types of analog information such as audio and video requires relatively high bandwidths. As another example, it might be desirable to provide a local area network for household computers and other computerized devices using existing power wiring. Higher bandwidths are required for these applications.
Unfortunately, components capable of transmitting at high bandwidths are relatively expensive to produce. This has become an impediment to widespread acceptance and use of any standard that allows high bandwidth communications such as required for audio, video, and computer networking applications.